In a cellular mobile communications system having a plurality of cells controlled by a plurality of base stations (e.g., a BTS (Base Transceiver Station), Node B or eNodeB), a user equipment (UE) generally communicates data or control signals with base stations, detects a cell that is suitable for the UE to receive normal and/or emergency services (hereinafter, such a cell will be referred to as “source cell”), and thereby can camp on that cell. “Camping on a cell” means that a UE in an idle mode connects to a cell found through cell selection/cell re-selection.
As used herein, “cell re-selection” is used when a user equipment, UE, is in e.g. an idle mode (IDLE, CELL/URA_PCH, EACH) and then the UE decides to re-select cell or not. In contrast, when referring to handover, it is referred to when the UE is in connected or dedicated mode (CELL_DCH). When in connected mode, measurement reports are sent to the network side, and the network side then decides to handover to another cell or not.
To merely present the background to the reader, in the Universal Mobile Telecommunications System (UMTS) system, a UE that has camped on a cell receives the identification of the cell, the identification of a tracking area, and a neighboring cell list through a broadcast channel (BCCH) from the serving base station controlling this cell, and monitors a paging channel (PCH). The UE also monitors pilot channels (CPICH) from neighboring cells periodically. Neighboring cell information included in the neighboring cell list typically includes the carrier frequencies of the neighboring cells and other system parameters.
When the UE has detected a better cell (generally, a cell exhibiting better radio quality) than the serving cell on which the UE is currently camping, then the UE can camp on this newly detected cell as its new serving cell. This cell re-selection may be performed by a UE, based on some criterion provided from the network side. For example, when the UE experiences deterioration in the radio quality of its serving cell, and if this state of deterioration continues for a certain period of time then the UE can perform in cell re-selection by using the neighboring cell information in the neighboring cell list. Such cell re-selection may be performed through the following procedure: While sequentially setting a carrier frequency according to the neighboring cell information stored in the neighboring cell list, the UE checks whether or not a neighboring cell having better radio quality than that of the current serving cell is detected. When such a cell having better radio quality is found (thus, detected), the UE selects and camps on this cell having better radio quality as its new serving cell. As described above, since it is sufficient for the UE to check and detect the radio qualities only of the neighboring cells stored in the neighboring cell list, it is possible to select a cell having better radio quality at a relatively high speed. Additionally, the neighboring cell list is also utilized to measure the radio qualities of the neighboring cells at the time of handover.
If the UE cannot detect a suitable cell among the cells of the neighboring cell list provided from the network side, the UE typically starts carrier sequential search, by which all possible carrier frequencies can be sequentially searched, to find a suitable cell to camp on. The detection of a suitable cell by means of this carrier sequential search is called “cell selection” in the UMTS system. The cell selection may include initial cell selection and stored-information cell selection. In the initial cell selection, the UE can sequentially search all existing frequency bands, thereby detecting a suitable cell. In the stored-information cell selection, the UE, having stored information about carrier frequencies in advance, carries out carrier sequential search of the frequency bands limited by this stored-in-advance information, thereby detecting a suitable cell. The stored-information cell selection typically enables higher-speed detection of a suitable cell because carrier sequential search is carried out in limited frequency bands.
In the cell selection, in any case, the UE carries out sequential search of carrier frequencies. Therefore, the cell selection generally requires a longer time than the cell re-selection, and services may become interrupted during this period of time.
Procedures for the cell selection and for the cell re-selection in the UMTS system are described in the following 3GPP (The 3rd Generation Partnership Project) technical specifications:                3GPP TS 25.133 “Requirements for support of radio resource management (FDD)” (Version 11.0.0, publicly available in January 2012);        3GPP TS 25.304 “User Equipment (UE) procedures in idle mode and procedures for cell re-selection in connected mode” (Version 10.3.0, publicly available in December 2011); and        3GPP TS 25.331 “Radio Resource Control (RRC); Protocol Specification” (Version 11.0.0, publicly available in January 2012).        
When performing a handover (in connected/dedicated mode) or cell re-selection (in idle mode), a conventional approach is to try to find all cells comprised in a neighboring cell list and measure on all the detected cells with equal priority. i.e. the frequency and accuracy of the measurements for all the cells is the same. When the number of cells is large, the UE may not get enough time to detect and measure on all cells and the handover decision may thus be made based on old measurement values. This means that the UE may not be handed over (in connected/dedicated mode), or re-select (in a non-connected/non-dedicated (e.g. idle mode), to the correct cells quickly enough, thereby increasing the risk of dropping a call.